1. Field of the Invention
The present invention relates to a superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called xe2x80x9cMRIxe2x80x9d). More particularly, the invention relates to an arrangement for improving the stability of the superconducting MRI magnets in the assembly. A method of making a superconducting magnet assembly is also provided.
2.The Prior Art
Many superconducting MRI magnets are wound with superconductor wires consisting of a superconductor core and a copper or aluminum stabilizer. When a superconducting magnet is energized at low temperatures, the electric current will flow through the wire""s superconductor core. The electric current results in an electromagnetic force that applies to the superconductor core. The force is reacted against the copper or aluminum stabilizer that is supported by the coil supporting structure.
Certain superconductor wires are made so that the superconductor core is soldered into a copper stabilizer channel. A circular coil is commonly wound with this type of wire so that the channel opening faces radially inward. With this orientation, the core is in the inner diameter and the channel covers the core on the outer diameter. This orientation transfers the radial electromagnetic force acting on the core to the channel through the bottom of the channel.
However, a problem arises when the radial electromagnetic force reverses. In certain types of magnet coil designs with multilayer wires, a very low coil aspect ratio is required. A low-aspect-ratio coil in a magnetic resonance magnet is a coil whose radial dimension is greater than or approximately the same as the axial dimension of the coil. In these designs, wires on the outer portion of the coil near the outer diameter could experience large radially inward force. In some other magnet designs, some of the coils have reverse current with respect to the main coil current. These coils are wound on the same main coil structure and may experience a huge net radially inward force. In these cases, the direction of force on the core tends to push the core out of the channel. As a result, the superconductor core separates from the stabilizer.
In response to the above problems, a superconducting magnetic resonance imaging magnet assembly is provided. In one aspect, the wire orientation is reversed for the outer diameter portion of the low-aspect-ratio coils in the main magnet coil assembly. In another aspect, the wire orientation is reversed for the reverse current coils on the main magnet coil assembly. These reverse current coils have very large radially inward force. The assembly comprises an axial imaging bore to receive patients and a coil housing surrounding the axial bore. A main magnet coil assembly is disposed within the housing. The main magnet coil assembly includes a plurality of main magnet coils, for example six, eight or ten coils, which are preferably circular, to produce a magnetic field. At least three pairs of superconductive coils are included in the main magnet coil assembly.
In one embodiment, each of the coils in at least one of the pairs of superconductive coils comprises a superconductor wire carrying an electric current in a direction opposite to the direction of current carried by the coils of another of the superconductive coil pairs. The superconductive wire includes a superconductor core and a stabilizer. The stabilizer comprises a channel having a radially outward facing opening.
In another embodiment, low-aspect-ratio coils are used for at least one of the pairs of superconductive coils. Each low-aspect-ratio coil comprises an inner diameter coil portion joined to an outer diameter coil portion. The inner diameter coil portion comprises a superconductor core and a stabilizer channel having a radially inward facing opening. The outer diameter coil portion comprises a superconductor core and a stabilizer channel having a radially outward facing opening. The two coil portions may be connected together with a superconducting joint or a resistive joint.
The stabilizer may be formed from copper or aluminum and the superconductor core soldered into the stabilizer channel.
In a further embodiment, a bucking coil assembly including at least two bucking coils, is disposed within the coil housing.
A method of making a superconducting magnetic resonance imaging magnet is also provided. In one aspect, a first main magnet coil is formed carrying electric current in a first direction. A second main magnet coil is formed carrying electric current in a second direction opposite to the first direction. Each second main magnet coil comprises a superconducting wire comprising a superconductor core and a stabilizer. The stabilizer comprises a channel having a radially outward facing opening.
In another aspect, a second main magnet coil having a low coil aspect ratio is formed. The inner diameter portion of the second main magnet coil is formed by winding a superconductor wire having a superconductor core and a stabilizer channel having a radially inward facing opening. The outer diameter portion of the coil is then formed by winding a superconductor wire having a superconductor core and a stabilizer channel having a radially outward facing opening.